This invention relates in general to field emission displays for electronic devices and, in particular, to improved cathodoluminescent layers for field emission displays.
FIG. 1 is a simplified side cross-sectional view of a portion of a display 10 including a faceplate 20 and a baseplate 21 in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface and forms a hermetically sealed package between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into pixels yielding different colors to provide a color display 10. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 include Y2O3:Eu (red, phosphor P-56), Y3(Al, Ga)5O12:Tb (green, phosphor P-53) and Y2(SiO5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a surface of a substrate 32, which may be a semiconductor such as silicon. Although the substrate 32 may be a semiconductor material other than silicon, or even an insulative material such as glass, it will hereinafter be assumed that the substrate 32 is silicon. The substrate 32 is coated with a dielectric layer 34 that is formed, in one embodiment, by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 30. This thickness may be on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be, for example, a thin layer of polysilicon. An opening 40 is created in the extraction grid 38 having a radius that is also approximately the separation of the extraction grid 38 from the tip of the emitter 30. The radius of the opening 40 may be about 0.4 microns, although larger or smaller openings 40 may also be employed.
In operation, the extraction grid 38 is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate 32 is maintained at a voltage of about zero volts. Signals coupled to the emitter 30 allow electrons to flow to the emitter 30. Intense electrical fields between the emitter 30 and the extraction grid 38 then cause emission of electrons from the emitter 30. A larger positive voltage, ranging up to as much as 5,000 volts or more but generally 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by this voltage and strike the cathodoluminescent layer 26. This causes light emission in selected areas, i.e., those areas adjacent to the emitters 30, and forms luminous images such as text, pictures and the like.
When the emitted electrons strike the cathodoluminescent layer 26, compounds in the cathodoluminescent layer 26 may be dissociated, causing outgassing of materials from the cathodoluminescent layer 26. When the outgassed materials react with the emitters 30, their work function may increase, reducing the emitted current density and in turn reducing display luminance. This can cause display performance to degrade below acceptable levels and also results in reduced useful life for displays 10.
Residual gas analysis indicates that the dominant materials outgassed from some types of cathodoluminescent layers 26 include hydroxyl radicals. The hydroxyl radicals reacting with the emitters 30 leads to oxidation of the emitters 30, and especially to oxidation of emitters 30 formed from silicon. Silicon emitters 30 are useful because they are readily formed and integrated with other electronic devices on the substrates 32 when the substrate is silicon. Electron emission is reduced when silicon emitters 30 oxidize. This leads to time-dependent and/or degraded performance of displays 10.
In conventional cathode ray tubes (xe2x80x9cCRTsxe2x80x9d), some scrubbing of the cathodoluminescent screen is typically carried out after the tube is sealed using an electron gun of the type contained in a CRT. xe2x80x9cScrubbing,xe2x80x9d as used here, means to expose the cathodoluminescent layers (e.g., cathodoluminescent layer 26) to an electron beam until a predetermined charge per unit area has been delivered to the cathodoluminescent layer 26. This scrubbing is carried out at a very low duty cycle and at a very low current density because the electron beam is rastered over the area of the cathodoluminescent screen. It is also carried out at the same current levels that the CRT is expected to support in normal operation, typically 100 microamperes/cm2 or less. However, this approach will not work for scrubbing cathodoluminescent layers 26 for the displays 10, in part because the emitters 30 in the displays 10 are poisoned by the chemical species evolving from the cathodoluminescent layer 26 in response to the scrubbing operation. Moreover, the cathodoluminescent layer 26 is typically much less than a millimeter away from the emitters 30, i.e., the mean free path for any gaseous chemical species evolving from the cathodoluminescent layer 26 is much larger than the distance separating the cathodoluminescent layers 26 from the emitters 30. In contrast, the electron gun used to scrub cathodoluminescent layers in a CRT are not adversely affected by this chemical species and electron guns are, as a rule of thumb, displaced from the cathodoluminescent screen by a distance approximately equal to the diagonal dimension of the CRT screen.
There is therefore a need for a technique to prevent evolution of oxygen-bearing compounds from cathodoluminescent screens in field emission display faceplates.
In accordance with one aspect of the invention, a low voltage, high current, large area cathode for electron scrubbing of cathodoluminescent layers is described. The electron scrubbing is particularly advantageous for use with cathodoluminescent screens of field emission displays having silicon emitters. The present invention includes an apparatus to irradiate a cathodoluminescent layer in a vacuum with an electron beam and a device to move the cathodoluminescent layer relative to the irradiating apparatus. The irradiation is stopped when a predetermined total Coulombic dose has been delivered to the cathodoluminescent layer. Significantly, the scrubbing results in a cathodoluminescent layer that does not outgas materials that are deleterious to performance of silicon emitters. This results in a more robust display and extended display life.